The foamed resin molding machine conventionally used in the field of foaming and molding is designed in a structure consisting of a fixed die plate and a movable die plate, for opening and closing the die plates by mutually approaching or departing by piston driving operation as disclosed, for example, in patent document 1. Specifically, a die opening and closing device as shown in FIG. 5 is used, in which a piston 511 is mounted on a movable die plate 2 having a movable die 4 attached to the front face, and this piston 511 is driven by a driving cylinder 521, and the movable die 4 is moved closer to or apart from a fixed die 3 attached to a fixed die plate 1.
In such foamed resin molding machine, corresponding to the product to be foamed and molded, since the die consisting of the fixed die 3 and movable die 4 is exchanged and mounted, and the die interval when the fixed die 3 and the movable die 4 contact with each other (hereinafter called the die thickness) S varies to be wider or narrower. When the die thickness S is narrower, if the stroke of the piston 51 is sufficiently large, the movable die 4 securely contacts with the fixed die 3. However, when the die thickness S is narrower than the stroke of the piston 51, the movable die 4 does not contact with the fixed die 2, and as shown in FIG. 5, a distance piece 531 is attached to the leading end of the piston 51 to compensate for shortage of the stroke. On the other hand, when the die is exchanged, and the die thickness S is extremely wider, the distance piece 531 must be removed. The distance piece 531 is attached or detached in a narrow working space, which is a troublesome task and the machine operator may be stained with oil.
In such a die opening and closing device, usually, in the process of moving the movable die plate 4 closer, to fill the cavity formed in the die with material beads (expandable resin beads), the operation is once stopped while keeping a specified cracking gap between them, and after filling with materials, the dies are moved closer and tightened. The cracking gap is very important as an air escape route when supplying the materials into the die by air flow.
However, in the die opening and closing device of the conventional hydraulic cylinder system, since the pressing force of the movable die plate 4 fluctuates depending on the oil viscosity, temperature, speed of the movable die plate 4, or weight of the movable die 4, control of stopping position of the movable die plate 4 is unstable, errors of ±0.5 to 1.0 mm occur in the cracking gap as compared with the predetermined value, and the molding conditions are not constant.
When demolding the molding from the die, the movable die plate 4 is opened by setting a cracking gap of 5 to 50 mm from the molding, and at this time, air is being blown into the die to lift the molding from the die. In the die opening and closing device of the conventional hydraulic cylinder system, due to deviation of air blowing pressure, the cracking gap may fluctuate, and errors of ±50 to 100 mm may occur from the predetermined cracking gap. Due to such large errors, the molding cannot be transferred to the opposite die, and the molding cannot be taken out smoothly. As a result, the molding may be deformed or damaged, or pierced by an eject pin.    Patent document 1: Japanese Patent Application Laid-Open No. 1993-154930    Patent document 2: Japanese Patent Application Laid-Open No. 1994-182888    Patent document 3: Japanese Patent Application Laid-Open No. 1998-180884    Patent document 4: Japanese Patent Application Laid-Open No. 1993-193014